Acid rain has been tackled for over a decade with some success.
Overview
Acid rain damages soils, vegetation, rivers and lakes. The problem has been tackled for over a decade with some success.
Acid rain is formed when industrial emissions of gases including sulphur dioxide, nitrogen oxides and hydrogen chloride combine with water droplets in the atmosphere. This chemical reaction forms weak sulphuric and nitric acids that cause acid rain.
Sulphur deposition levels have fallen in recent years (
Figure 1). Sulphur dioxide emissions decreased by 78% from 1987 to 2004, which led to 82% less dry deposition and 50% less wet deposition.
Sulphur levels have fallen because power stations have invested in technologies that remove sulphur from flue gases and use alternative, less sulphurous fuel sources such as natural gas. Also there are international agreements in place to reduce air pollutant emissions.
Acidic pollutants can be deposited in three different ways:
- wet deposition - pollutants are deposited in rain and snow (commonly termed "acid rain"). This happens mainly in upland areas where rainfall is highest.
- dry deposition - gases and particles are deposited directly onto the land. This is greater than wet deposition in many parts of the UK
- cloud deposition - clouds can deposit acidic pollutants over high ground.
Sulphur dioxide and nitrogen oxides are relatively insoluble. This means that they might not combine with water in the air to make acid rain until they have travelled a long way from their source. For example, sulphur dioxide released into the air in the UK can be blown hundreds of miles away before it falls as acid rain on countries such as Canada, Norway, Sweden and Finland.
In the UK, most sulphur is deposited close to the area of highest man-made emissions - the Midlands. Nitrogen is deposited over a large area because a lot of nitrogen oxides come from traffic. High deposits of both sulphur and nitrogen are found where rainfall is highest, such as northern England and the Welsh hills.
Different parts of the environment respond differently to acid rain, and some are more easily harmed than others.
Acid rain can upset the chemical balance in rivers and lakes, killing fish and other organisms. It can also damage plants, trees and buildings.
Effect on soils
Soils on chalk and limestone are naturally alkaline and neutralise acidic deposits. Naturally acidic soils, like those found in much of Cumbria and Snowdonia, are more sensitive to acidic deposits.
Effect on plants
Nitrogen deposits (from nitrogen oxides) can act to fertilise the soil. This can alter the soil’s chemistry, which in turn alters the plants that normally grow on it. Changes in plant communities can then affect the wildlife living on them. At the same time acid rain also releases toxic substances such as aluminium into the soil. Even a limited exposure can be very harmful.
Acid rain can damage plants and trees. Leaves feed plants through photosynthesis (turning sunlight into food). A tree may not produce enough food to remain healthy if its leaves are damaged. A weakened tree is more likely to suffer from disease, insect attacks and cold weather.
Effect on rivers and lakes
Acid rain can acidify the water bodies it falls into. Acidified water releases aluminium from the soil that the lake covers. The released aluminium can be toxic to plants, fish and insects. In Sweden, acid rain has made over 18,000 lakes so acidic that all the fish have died. Acid rain has affected some salmon and trout fisheries in small headwater rivers in Wales. Natterjack toads in the south of England may have been lost due to the their spawning ponds becoming more acidic, and dippers (a river bird) have lost some of their food supply.
Environmental facts and figures on
